The invention relates generally to circuits for performing signal analysis and more particularly to methods of determining operational parameters for such circuits.
Signal analysis instruments are used in designing and testing individual components and completed systems. One conventional signal analysis instrument is the oscilloscope, which may be used to determine measurements of signal variations as a function of time. Signal analysis in the time domain provides useful information, but frequency-domain analysis is at least equally important in some situations. For example, telecommunications systems which use Frequency Division Multiplexing (FDM) techniques may be more thoroughly tested by using instruments which yield frequency domain information. Spectrum analyzers are often used to acquire frequency-domain measurements relating to levels of modulation, distortion and noise.
In a spectrum analyzer having one frequency conversion to a lower frequency level, the test signal to be analyzed is connected as one input to a mixer of the instrument. At the mixer, the test signal is combined with the signal of a local oscillator (LO), so as to provide an output at an intermediate frequency (IF). The IF is the tuned frequency of the analyzer. The down converted signal is propagated through an IF filter. The filter output provides a measure of the presence of a signal component at the analyzer""s tuned frequency. In order to improve performance and/or capability, some instruments include two frequency conversions. The filtered output of the first IF filter is an input to a second mixer, which has a second input from a second local oscillator. As a result, a second IF is provided at the output of the second mixer. Merely as one example, the first IF may be 3245 MHz (3.245 GHz), while the second IF may be 135 MHz. Thus, a xe2x80x9cfrequency plan,xe2x80x9d which defines internal operational parameters of a circuit, will be defined in accordance with the original circuit design that includes the two IFs to which the test signal is converted.
An important element of a circuit that includes one or more conversions to a final IF is the first IF filtering. In a multi-conversion circuit, the first IF filtering needs to provide high frequency selectivity, so as to reject image signals before they reach the second conversion. Each mixer will include a number of different signals at its output. In addition to outputting the two signals being mixed, the available mixer output signals include the sum frequency signal and the difference frequency signal. While not critical, the center frequency of the passband of the IF filter following a mixer is typically the difference frequency signal of the mixer.
A number of filtering techniques have been used to provide the first IF filtering. For example, cavity resonators and dielectric resonators have been used. Another approach is to use printed stripline interdigital filters, since such filters provide the desired high selectivity without introducing an unacceptable level of loss. The concern with printed stripline interdigital filters is that there are significant differences in the filtering characteristics of such filters from one fabrication lot to the next, and even in the same fabrication lot. Significant variations in the filtering characteristics may lead to a reduction in the fabrication yield, since some fabricated circuits may not reach minimum quality control standards. Moreover, while other fabricated circuits may satisfy quality control requirements, the performance may be less than optimal.
What is needed is a method for controlling the effects of circuit-to-circuit variations in the filtering characteristics of signal analyzer circuitry having input signal frequency conversion,
Frequency-related operational parameters of a signal analysis circuit having frequency conversion are selected on the basis of factors that include intermediate frequency (IF) filtering characteristics that are uniquely identified for the circuit. By tailoring a frequency plan to the actual IF filtering characteristics, rather than the target IF filtering characteristics of the circuit designer, a circuit may be used even when there is a significant difference between actual and target IF filtering characteristics. Consequently, rather than discarding a circuit board that does not meet specifications, a frequency plan may be individualized to enable passage of the desired signal components while inhibiting passage of externally generated and internally generated spurious responses.
In a typical application of the invention, the circuit of concern includes at least two frequency conversions. As one example, the circuit may be formed as a printed circuit board for a spectrum analyzer that uses heterodyne techniques. However, other applications of the invention are contemplated.
In a first step, a signal analysis circuit design and printed circuit board layout are selected. At least one circuit is fabricated in accordance with the design and layout. Typically, a number of such circuits are fabricated, either in a single lot or a number of different lots. The circuit may include first and second frequency conversions to first and second intermediate frequencies, respectively, with a first IF filter between the two conversions.
Each signal analysis circuit is tested to determine the actual IF filtering characteristics of the first IF filter. A particularly important characteristic is the center frequency of the passband defined by the filter.
With the identification of the IF filtering characteristics, the frequency plan for the circuit may be selected. For example, a circuit having two frequency conversions may be set according to the equations: LO1=RF+IF1 and LO2=IF1xc2x1IF2, where LO1 and LO2 are the frequencies of the local oscillators for the conversions, IF1 and IF2 are the two intermediate frequencies that result from the conversions, and RF is the input signal under test.
A simplified frequency plan does not consider externally generated spurious responses, which are often referred to as crossing spurs. Crossing spurs are due to nonlinearities inherent in receiver components, such as mixers and amplifiers. A high performance signal analysis circuit considers crossing spurs in selecting a frequency plan. Crossing spurs can be calculated using the equations IF2=xc2x1M*IF1xc2x1N*LO2 and IF2=M*IF1xc2x1N*LO2. The crossing spurs may be avoided within the output of the signal analysis circuit by selecting a frequency plan that inhibits low order products.
Internally generated spurious responses are more problematic. Such spurious responses are often referred to as residuals, since they are present with the termination of the input signal to the circuit. While crossing spurs are loosely specified, residuals are often tightly specified. Within the selected frequency plan, the operational parameters of the second conversion can be designed to avoid residuals by using the high side or the low side of the IF, depending on the position of the residual. Often, this can be accomplished by flipping the conversion so as to avoid residuals. The residuals can be located by the equation IF=xc2x1M*LO1N*LO2. Then, the frequency of the second local oscillator may be established according to the equation LO2=IF1+IF2 or LO2=IF1xe2x88x92IF2. This may be less than a ten percent change in the frequency of the second local oscillator, which is unlikely to cause a reduction in other aspects of signal analysis performance.
For circuits that use fractional-N synthesizers for controlling local oscillators, spurious responses are generated whenever the phase accumulator of the synthesizer completes a cycle. A loop bandwidth of approximately 10 KHz will be considered. Any spurious response generated outside of the loop bandwidth of a fractional-N synthesizer will have some attenuation, assuming power supplies are adequately filtered. These xe2x80x9cstructurexe2x80x9d spurs are preferably outside the bandwidth by a minimum of 50 KHz. The structure spurs occur when the .f numbers are near integer values. For example, for a .f of 0.002, a structure spur will occur every 1/0.002 cycles, or 500 cycles. With a reference signal of 20 MHz for the synthesizer, the spur frequency would be (0.002) (20 MHz)=40 KHz. To achieve the 50 KHz minimum, .f values of less than 0.0025 and greater than 0.9975 should be avoided. This can be accomplished by moving the frequencies of the first and second local oscillators by a small amount when the .f values to be avoided are reached.
An advantage of the invention is that there is a reduction in the number of situations in which a circuit needs to be redesigned or a fabricated printed circuit board needs to be replaced. Following circuit fabrication, the center frequency of the first IF filter may be identified by stepping the frequencies of the local oscillators and monitoring the output of the circuit. After the center frequency is identified, the characteristics of spurious responses may be determined. As one possibility, a spur calculation program may be used to identify the high/low switch points relevant to the frequency plan for the second conversion. The calculated switch points can then be stored in memory, such as by non-volatilely storing the data into flash memory. By uniquely identifying IF, for each circuit, it is possible to calculate target frequencies for avoiding spurious responses. Crossing spurs can then be predicted and measured in order to verify specifications.